1. Field of the Invention
The invention relates to an optical data transmission system. It is particularly, but not exclusively, related to a system comprising a passive optical fibre network.
2. Description of Related Art
In order to provide broadband telecommunications, optical transmission systems have been developed. Typically such systems transmit optical signals along optical fibre in the wavelength range of 1300 to 1600 nm. In order to provide multiple access for a plurality of users, it has been proposed for optical transmission systems to use wavelength division multiple access (WDMA).
An optical transmission system 110 is shown in FIG. 1. The system 110 comprises a central office (CO) or hub 112 connected to groups of user devices or optical network units (ONUs) 114. In this embodiment only one group 116 is shown although it will be understood that there would be a plurality of such groups. The hub 112 is connected by a common optical fibre 118 to a wavelength division multiplexer (WDM) 120. The user devices 114 are connected separately to the WDM 120 by respective optical fibres 122. The WDM 120 is said to be at a kerb location.
A downstream direction is defined from the hub 112 to the user devices 114 and an upstream direction is defined from the user devices 114 to the hub 112. Data traffic is transmitted both upstream and downstream to and from the user devices 114. The data traffic is in the form of modulated light as will be explained in the following.
Communication in the system occurs across a wavelength range, typically 1530 to 1565 nm. Each user device is assigned a wavelength band, typically spaced by 0.8 nm, with which to communicate. The hub 112 is equipped with individual laser sources producing light at different wavelengths suitable for different user devices. In the downstream direction, the hub 112 forms data traffic by modulating data onto appropriate light wavelengths which are then transmitted down the common optical fibre 118 to the WDM 120. The WDM 120 separates the data traffic according to wavelength and then directs it to particular user devices 114.